U.S. Pat. No. 4,618,732 provides a process for the direct conversion of natural gas to methanol. This is achieved by reacting natural gas with oxygen or air in the absence of a catalyst in an inert reactor at an elevated temperature and pressure. To obtain a high yield of methanol, the reactant gases, i.e. natural gas and oxygen or air, are first intimately mixed. In another aspect, said prior art provides apparatus for carrying out a method of converting natural gas to methanol. The apparatus comprises an inert reactor and means for intimately mixing the oxygen or air and natural gas prior to their introduction into the reactor. Means are also provided to ensure that the gases reach at suitable elevated temperature and pressure in the reactor. By the term “inert reactor” is meant a reactor that has internal surfaces made of, or coated with, a material which has no substantial adverse effect upon the methanol yields or selectivity. Preferably, the reactor is made of stainless steel lined with glass or polytetrafluoroethylene. The pressures employed in the reactor are generally in the range of 10 to 100 atmospheres, more preferably 10 to 60 atmospheres, and even more preferably 10 to 50 atmospheres. The temperature employed in the reactor is generally in the range of 300° C. to 500° C., more preferably 350° C. to 450° C. The contact time of the gases depends to some extent on the temperature, pressure and relative oxygen concentration, but the normal contact time is within the range of 2 to 1000 seconds, preferably 5 to 15 seconds, and more preferably about 10 seconds. According to FIG. 5 at 350° C. the yield of CO and CO2 are similar. According to FIG. 6 at 65 atmospheres and 410-430° C. the ratio of CO to CO2 is about 2.
U.S. Pat. No. 4,982,023 describes the synthesis of methanol by the homogeneous direct partial oxidation of natural gas or other source of methane when the reactor space is filled with inert, refractory inorganic particles. The reactor is a 16.5 mm i.d. Pyrex-lined tube. Both the yield and the selectivity in the direct homogeneous partial oxidation of a gaseous feed comprising methane and gaseous oxygen are improved when the empty reactor is packed with a low surface area solid such as sand. The runs were performed using natural gas feed comprising 95.66 w % methane.
Example 1 is made with an empty tube at 68 bars, 360° C., 6.4% O2 in the feed and 4 minutes residence time. Conversion is 5.5%, CO selectivity 49.4%, CO2 selectivity 21.8%, methanol selectivity 25.8% and other oxygenates 3%.
Example 2 is made with a tube filled with sand, at 68 bars, 400° C., 7% O2 in the feed and 4 minutes residence time. Conversion is 5.9%, CO selectivity 40%, CO2 selectivity 21.7%, methanol selectivity 27.2% and other oxygenates 11.1%.
WO 00-007718 describes a catalytic composition, optionally supported on an inert material, characterized in that it comprises (i) oxides and/or hydroxides of a first metal (M1) and (ii) halides of a second metal (M2), wherein M1 and M2, the same or different, are selected from metals belonging to groups IIa, IIb, IVb, VIII, Ib, Va, Lanthanides, and relative mixtures. It also relates to the selective transformation of methane on said catalytic composition, in example 7 the reactor is made of quartz.
U.S. Pat. No. 4,918,249, GB 1244001, U.S. Pat. No. 5,414,157 and GB 1398385 also relate to the oxidation of methane on catalysts.
It has now been discovered that the methane oxidation to methanol could be made in a silica tube, advantageously a quartz tube, advantageously empty. Advantageously the quartz tube is HF treated.                a quartz-tube that is treated with HF aqueous solution is more active and selective in the selective methane oxidation into mainly methanol and carbon monoxide. Only small amounts of formaldehyde and carbon dioxide are produced.        it is preferable that the quartz reactor tube is empty. When filled with quartz particles, the methane conversion is significantly reduced.        when smaller diameter quartz tubes are used the conversion and selectivity increases, so the surface-to-volume ratio appears to be important.        